Alvarus leggings consits of having naxos faction (get it on center island in meso) talking to casydra (loc 45k 38k meso) killing razor fin (loc is sw of phoebus harp encounter hes a roamer so gotta look a little bit) and giving fin back to casydra.

====================================EDIT=======================================It is no longer required you turn in the fin. all you do is talk to casydra, go kill razorfin, and you get credit. the return of the fin gives you casydra's key which opens a chest that used to contain the artifact. now it contains a random item. ===============================================================================

Just a guess so please no flames,if ya know ya should be posting, anyways i killed razorfin a few times in the skyros area. He has always dropped a set of chain boots and razor's fin. That and the line "when female with sky is friend" leads me ta think this thing is related. There's that female gin on the harpy island that allied with the skyros. Just food for thought. Hope it helps someone peice the rest together. Till then the fin collects dust in the vault.